1. Technical Field
The present invention relates to telecommunications networks interconnected using optical fibers and, more particularly, to transmission of a supervisory channel among multiple traffic channels carried by such optical fibers.
2. Discussion of Related Art
Fiber optic transmission systems are capable of operating multiple channels on a single fiber using wavelength-division multiplexing (WDM). Some systems transmit all WDM channels in one direction on the fiber, so that two fibers are needed to achieve duplex communication. Bidirectional operation, on the other hand, can operate in both directions on a single fiber. The problem is that presently available optical supervisory channels (OSC) can only operate in one direction on the fiber. (The OSC is used for system maintenance and alarm reporting to/from remote sites.)
The reason for this shortcoming is that existing approaches use an OSC on one wavelength, such as 1310 nm or 1480 nm. But these techniques require two fibers to obtain duplex communication. Another simplex approach uses 1532 nm for an OSC, and inserts and drops it between the stages of a two-stage optical amplifier.
These approaches cannot obtain duplex communication on one fiber with bidirectional supervisory signals. In addition, the last-mentioned simplex approach using 1532 nm has the disadvantage that the 1532 nm OSC occupies a region of the optical amplifier that can be used for revenue-producing traffic channels.
The object of the present invention is to achieve duplex communications using wavelength division multiplexing on a single fiber with appropriate supervisory channels in both directions and to do so in an efficient manner.
According to a first aspect of the present invention, a method for communicating an optical supervisory channel (OSC) in an optical fiber network, comprises the steps of receiving, in a first network element, a first optical supervisory channel signal at a first wavelength from a first direction on a first optical fiber (16), and transmitting, from said first network element, a second optical supervisory channel signal at a second wavelength in a second direction (20) on said first optical fiber. Thus, according to the present invention, two wavelengths are used for the OSC, one for each direction on the fiber.
In further accord with the present invention, the first and second optical supervisory channel signals are transmitted with revenue traffic using wavelength-division-multiplexing. The first and second wavelengths are selected for passing through an optical amplifier in an amplified optical wavelength division multiplexed transmission system. Both the first and second wavelengths are located outside of a flat-gain amplifying region of the optical amplifier.
The OSC wavelengths are thus located where they can pass through an optical amplifier, but not occupy the xe2x80x9cflat-gainxe2x80x9d region that can be used for revenue-producing traffic. One example is to locate them in the regions slightly outside the flat-gain region where there is some gain, but is not otherwise usable. The first wavelength can reside below a revenue-traffic amplifying region of the optical amplifier and the second wavelength can reside above the revenue-traffic amplifying region, with both the first and second optical wavelengths in a low-loss window of the first optical fiber.
The present invention advantageously permits operation of a duplex OSC over a single fiber, saving the cost of a second fiber. Choosing wavelengths that do not consume space in the xe2x80x9cflat-gainxe2x80x9d region allows additional revenue-producing traffic channels.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing.